1. Field of the Invention
The present invention relates to cathode devices. More specifically, the invention relates to thermionic microcathodes having integrated extractor electrodes. According to the invention, a cathode emits electrons into a via through a substrate such that the electrons pass through the entire substrate, then through an aperture in an extractor electrode, and towards an anode. The microcathode device of the invention is particularly suitable for use with various types of electron beam equipment such as flat cathode ray tube displays, microelectronic vacuum tube amplifiers, and other such electron beam exposure devices and the like.
2. Description of the Related Art
It is known in the field of electron beam emitting devices to place a cathode at a negative potential relative to an anode. Typically, with cathode ray tubes or the like, electron emission is achieved by heating the cathode to a sufficiently high temperature that electrons have enough thermal energy to be emitted from the cathode. The potential difference between the cathode and the anode accelerates the emitted electrons from the cathode towards the anode in the form of an electron beam. This technology has been used in various devices, such as cathode ray tube displays, electron microscopes and the like.
One major technical challenge in the field of electron emissions relates to the tendency of emitted electron beams to disperse at an angle on the order of 30 degrees. Such a dispersion spreads the beam over a relatively wide area, resulting in a image display of poor resolution. Many focusing schemes have been proposed to reduce the dispersion of electrons as they traverse the space between the emitting cathode and collecting anodes. See, for example, U.S. Pat. No. 5,070,282 which discloses the use of a negatively biased control electrode which causes electrons to converge toward the axis of the beam. See also U.S. Pat. No. 5,235,244 which discloses a passive dielectric electron beam deflector.
Cathode devices using separate extractor electrodes to provide beam focusing are known in the art. However, when the cathode is smaller than about 1 mm in size, use of a separate extractor electrode presents difficulties in assembly and precise alignment with the cathode. These difficulties result in increased production costs and compromised performance. It would be desirable to devise a more economical microcathode device which integrates both a cathode and an extractor electrode, and which provides simplified fabrication and self-alignment of the cathode and extractor. A smaller device size also provides benefits of lower cathode heater power, lower cost, and application to devices requiring very small cathodes.
The use of extractor electrodes is described in C. A. Spindt, xe2x80x9cA Thin-Film Field-Emission Cathodexe2x80x9d, J. Appl. Physics, Vol. 39, pp. 3504-3505, 1968; P. R. Schwoebel and C. A. Spindt, xe2x80x9cField-Emitter Array Performance Enhancement Using Hydrogen Glow Dischargesxe2x80x9d, Appl. Phys. Lett., vol. 63, pp. 33-35, 1993. Spindt and Schwoebel describe a field emitter microcathode having an aperture grid fabricated from patterned thin films. However, these references greatly differ in arrangement from the present invention, and do not include thermionic cathodes.
Thermionic microcathodes are described in C. C. Perng, D. A. Crewe, A. D. Feinerman, xe2x80x9cMicromachined Thermionic Emittersxe2x80x9d, J. Micromech. Microeng., Vol. 2, pp. 25-30, 1992. Perng et al describes a micromachined narrow tungsten wire which acts as a thermionic microcathode. However, unlike the present invention, Perng et al do not describe the use of an integrated extractor or grid electrode. Furthermore, Perng et al. teach the use of tungsten, which requires much higher temperatures for thermionic electron emission than the materials of the present invention.
The present invention provides a thermionic microcathode which integrates both an electron emitter, or cathode, and an extractor electrode. The electron emitter comprises a low work function material and is attached to the back side of a thin film microstructure which has been formed on a first surface of a substrate. An electron beam is emitted from the electron emitter and into a via which extends through the substrate. The electron beam is pulled through the via and out of the microcathode by an extractor electrode on a second surface of the substrate. The extractor electrode defines the beam profile. By applying a variable voltage to the extractor, it can also modulate the electron beam current and provide a portion of the electric field needed to accelerate the electrons toward the anode located outside of the microcathode. An important advantage of the invention is that it can be fabricated at lower cost than conventional techniques in which the extractor and cathode are fabricated separately and subsequently assembled. Furthermore, the monolithic fabrication of the extractor and cathode on a single substrate allows self-alignment of these components. The invention results in significant cost savings while also enabling the fabrication of smaller and less complicated devices.
The invention provides a microcathode comprising a planar substrate having first and second opposite surfaces; a substrate via through the substrate which extends through the second surface of the substrate and a distance through the substrate toward the first surface; an electron emitter at a bottom of the via having an electrical connection through the bottom of the via; an extractor electrode at the second surface of the substrate which spans a portion of the via, which extractor electrode has at least one aperture adjacent to the via and opposite to the electron emitter, which extractor electrode is capable of controlling electrons emitted by the electron emitter through the aperture.
The invention further provides a microcathode comprising a planar substrate having first and second opposite surfaces; a plurality of substrate vias through the substrate which extend through the second surface of the substrate and a distance through the substrate toward the first surface; a plurality of electron emitters, one at a bottom of each via, having an electrical connection through the bottom of each via; and an extractor electrode at the second surface of the substrate which spans a portion of each via, which extractor electrode has an aperture adjacent to each via and opposite to each electron emitter, which extractor electrode is capable of controlling electrons emitted by each electron emitter through its corresponding aperture.
The invention still further provides an array of adjacent microcathodes, each microcathode comprising a planar substrate having first and second opposite surfaces; a substrate via through the substrate which extends through the second surface of the substrate and a distance through the substrate toward the first surface; an electron emitter at a bottom of the via having an electrical connection through the bottom of the via; an extractor electrode at the second surface of the substrate which spans a portion of the via, which extractor electrode has at least one aperture adjacent to the via and opposite to the electron emitter, which extractor electrode is capable of controlling electrons emitted by the electron emitter through the aperture.
The invention still further provides a microcathode comprising:
a) a substrate having first and second opposite surfaces;
b) an optional sacrificial material layer on the first surface of the substrate;
c) a thin film microstructure on the first surface of the substrate or on the sacrificial material layer, if present, which thin film microstructure has a back side facing the direction of the substrate and a front side facing away from the substrate;
d) a substrate via through the substrate which via extends through the first and second surfaces of the substrate and the sacrificial material layer, if present, such that the back side of the microstructure faces the substrate via;
e) an electron emitter on the back side of the thin film microstructure such that the electron emitter faces the substrate via;
f) an extractor electrode on the second surface of the substrate and spanning the substrate via, which extractor electrode has at least one aperture adjacent to the substrate via and opposite to the electron emitter, which extractor electrode is capable of controlling electrons emitted by the electron emitter through the aperture.
The invention still further provides a microcathode comprising:
a) a substrate having first and second opposite surfaces;
b) a sacrificial material layer on the first surface of the substrate;
c) a thin film microstructure on the sacrificial material layer, which microstructure has a back side facing the sacrificial material layer on the substrate and an opposite front side facing away from the substrate;
d) a substrate via through the substrate, which via extends through the first and second surfaces of the substrate and through the sacrificial material layer such that the back side of the microstructure faces the substrate via;
e) an electron emitter on the back side of the thin film microstructure facing the substrate via; and
f) an extractor electrode on the second surface of the substrate, which extractor electrode has at least one aperture adjacent to the substrate via and opposite to the electron emitter, which extractor electrode is capable of controlling electrons emitted by the electron emitter through the aperture;
wherein the microstructure comprises:
i) an insulator layer on the sacrificial material layer;
ii) an optional electron emitter contact layer on the insulator layer and in contact with the electron emitter;
iii) a heater filament layer on the insulator layer or on the electron emitter contact layer, if present;
iv) an optional additional insulator layer on the heater filament layer; and
v) at least two conductive contact pads electrically connected to the heater filament layer.
The invention still further provides a method for forming a microcathode which comprises:
a) providing a substrate having first and second opposite surfaces;
b) forming a sacrificial material layer on the first surface of the substrate;
c) forming a thin film microstructure on the sacrificial material layer, which microstructure has a back side facing the sacrificial material layer on the substrate and a front side facing away from the substrate;
d) forming a substrate via through the substrate which via extends through the first and second surfaces of the substrate and through the sacrificial material layer such that the back side of the microstructure faces the substrate via;
e) forming an electron emitter on the back side of the thin film microstructure facing the substrate via; and
f) forming an extractor electrode on the second surface of the substrate, which extractor electrode has at least one aperture adjacent to the substrate via and opposite to the electron emitter, which extractor electrode is capable of controlling electrons emitted by the electron emitter through the aperture;
wherein the microstructure comprises:
i) an insulator layer on the sacrificial material layer;
ii) an optional electron emitter contact layer on the insulator layer and in contact with the electron emitter;
iii) a heater filament layer on the insulator layer or on the electron emitter contact layer, if present;
iv) an optional additional insulator layer on the heater filament layer; and
v) at least two conductive contact pads electrically connected to the heater filament layer.
The invention still further provides a method for emitting electrons from a microcathode toward an anode which comprises:
I) providing a microcathode which comprises:
a) a substrate having first and second opposite surfaces;
b) a sacrificial material layer on the first surface of the substrate;
c) a thin film microstructure on the sacrificial material layer, which microstructure has a back side facing the sacrificial material layer on the substrate and an opposite front side facing away from the substrate;
d) a substrate via through the substrate, which via extends through the first and second surfaces of the substrate and through the sacrificial material layer such that the back side of the microstructure faces the substrate via;
e) an electron emitter on the back side of the thin film microstructure facing the substrate via; and
f) an extractor electrode on the second surface of the substrate, which extractor electrode has at least one aperture adjacent to the substrate via and opposite to the electron emitter, which extractor electrode is capable of controlling electrons emitted by the electron emitter through the aperture;
wherein the microstructure comprises:
i) an insulator layer on the sacrificial material layer;
ii) an optional electron emitter contact layer on the insulator layer and in contact with the electron emitter;
iii) a heater filament layer on the insulator layer or on the electron emitter contact layer, if present;
iv) an optional additional insulator layer on the heater filament layer; and
v) at least two conductive contact pads electrically connected to the heater filament layer; and
II) heating the heater filament layer and causing a flow of electrons from the electron emitter through the aperture in the extractor electrode toward an anode and controlling the flow of electrons through the aperture by the extractor electrode.
The invention still further provides a cathode which comprises a support, a metallic electron emitter on the support, which emitter has a layer of a low work function composition, of from about 0 to about 3 electron volts, on the emitter; and which emitter is electrically connected to a voltage source; a heater which is substantially uniformly positioned around and separated from the emitter and which heater is electrically connected to a voltage source.